Stan and Leon; In regards to your comments about passive mixers having
resulting in recievers with "very high noise figures";
"Active vs. Passive
I seem to remember in KK7B's articles on the T2/R2 using the SBL that the
end result was a very high noise figure. I would think that an active
mixer would not have the same problem. ?"
The SBL-1 by itself has a noise figure of about 7 dB which varies
somewhat with frequency, but 7dB is a good engineering number to use.
Depending on how input filtering is done and how the device is terminated
at the IF port, a "front end" noise figure of between 10 and 15 dB can be
achieved using tried and true circuits from "Solid State Design" and the
"Handbook". A look at the level of external noise at 7MHz shows that this
noise figure is adequate for even quiet locations if a typical antenna
(dipole, vertical) is used. deVoldere, ON4UN has a graph of external
noise levels as a function of frequency in his "Antennas and Techniques
for Low band DXing" from which this information can be derived. My old
RSGB handbook also has this information. I don't know why such a graph is
not included in the ARRL Handbook as it is a starting point for receiver
design.
With the high level 7 MHz signals generated by SW broadcasters in Europe
a receiver with no more gain in the front end than is needed seems to be
a good choice. The SBL-1 seems to fit the bill nicely. Termination has
been discussed in a number of forums and is rather straightforward,
although not minimum parts count.
I read somewhere that a golden rule of receiver design is; "gain in a
receiver at one frequency should be accompanied by an equal amount of
suppression (filtering) at other frequencies".
I think that KK7B commented that the noise figures he achieved with the
R-1 and R-2 were a few dB higher than he expected which he attributed to
1/f noise in the mixer at low (audio frequencies). This issue was
discussed further in a later QST Technical Correspondence which I could
not find in my rapid search. I seem to recall that the problem was not
in the mixer, but rather in the termination, and the author of the
technical correspondence suggested an improvement. I do not think it
would be a problem for using the SBL-1 at higher IF frequencies.
Good luck with your project and let us know how it turns out. - Duffey
KK6MC/5
James R. Duffey KK6MC/5 DM65
30 Casa Loma Road
Cedar Crest NM 87008
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